Tracking 'Apolar' NMe_4~+ Ions within Two Polyoxothiomolybdates that Have the Same Pores: Smaller Clathrate and Larger Highly Porous Clusters in Action

摘要

Two nanosized polyoxothiometalates were synthesized based on linking oxomolybdate building blocks with {Mo_2O_2S_2}~(2+) groups. Remarkably, both compounds are formed selectively primarily upon changing the related concentrations in a logical way; they exhibit common structural features based on the same {Mo_9O_6S_3}-type pores, which result in connections between {Mo_6O_(21)} pentagons and {Mo_2O_2S_2}~(2+) linkers. Whereas the much larger spherical Mo_(132)-type Keplerate contains twenty pores, the smaller Mo_(63)-type cluster remarkably contains only two. The two compounds and a similar Keplerate exhibit interesting supramolecular properties related to interactions with the unusual predominantly apolar NMe_4~+ cations. Structural characterization of the Mo63-type compound reveals in the solid state a clathrate-like species that contains four NMe_4~+ cations embedded in two types of structurally well-adapted pockets. Related NMR spectroscopic investigations in solution using NMe_4~+ as the NMR spectroscopic probe are in agreement with the solid-state description. ~1H NMR spectroscopic experiments (1D variable-temperature, 2D total correlation spectroscopy (TOCSY), exchange spectroscopy (EXSY), and diffusion-ordered spectroscopy (DOSY)) feature firmly immobilized and mobile NMe_4~+ ions in relationship with the type of host–guest arrangements. The use of the ~1H NMR DOSY spectroscopic methodology has been successfully applied to track the interactions of the NMe_4~+ cations with the {Mo_9O_6S_3} pores of a sulfurated Keplerate, thereby allowing the first quantitative analysis of this type of plugging process. The stability constant K=(210±20) mol~(-1) L is discussed related to the character of the process.